Mild hydrocracking of heavy hydrocarbon feedstocks employing silica-alumina catalysts

ABSTRACT

A mild hydrocracking process for the hydrodemetallation (HDM), hydrodesulfurization (HDS) and hydroconversion (HC) of hydrocarbon feedstocks such as residuum feedstocks which provides increased conversion of the 1000° F.+ hydrocarbon fraction to the 1000° F.- fraction and increased yields of middle distillates is disclosed. The process utilizes a catalyst comprising about 2.0 to about 6.0 wt. % of an oxide of a Group VIII metal, about 12.0 to about 25.0 wt. % of an oxide of molybdenum and 0 to about 3.0 wt. % of an oxide of phosphorus supported on a porous alumina support containing about 4.0 to about 30 wt. % of silica.

This is a division, of application Ser. No. 07/890,206, filed May 29,1992, now U.S. Pat. No. 5,320,743.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for mild hydrocracking of heavyoils. More particularly, this invention relates to a catalytic processfor converting heavy oils boiling above 650° F., such as vacuum gas oil(VGO) and VGO containing a high proportion of vacuum resid (VR) tolighter distillate products boiling at or below 650° F.

In the mild hydrocracking process of this invention a sulfur- andmetal-containing hydrocarbon feedstock, such as residua containing heavyoils, is contacted at an elevated temperature with hydrogen and acatalyst composition comprising a specified amount of a Group VIIImetal, such as an oxide of nickel or cobalt, a specified amount of anoxide of molybdenum and, optionally, a specified amount of an oxide ofphosphorus, such as phosphorus pentoxide supported on a poroussilica-containing alumina support. In the catalytic mild hydrocrackingprocess of this invention the sulfur- and metal-containing hydrocarbonfeed is contacted with hydrogen and the catalyst, which has a specifiedpore size distribution, in a manner such that a substantially higherconversion of the 1000° F.+ fraction of the hydrocarbon feed to the1000° F.- lighter products is achieved over that obtained with the useof prior art hydroprocessing catalysts while at the same time highlevels of sediment formation are avoided.

2. Prior Art

U.S. Pat. No. 4,941,964, incorporated herein by reference, discloses aprocess for the hydrotreatment of a sulfur- and metal-containinghydrocarbon feed which comprises contacting the feed with hydrogen and acatalyst in a manner such that the catalyst is maintained at isothermalconditions and is exposed to a uniform quality of feed. The catalyst hasa composition comprising 3.0-5.0 wt. % of an oxide of a Group VIIImetal, 14.5-24.0 wt. % of an oxide of a Group VIB metal and 0-2.0 wt. %of an oxide of phosphorus supported on a porous alumina support, and thecatalyst is further characterized by having a total surface area of150-210 m² /g and a total pore volume (TPV) of 0.50-0.75 cc/g with apore diameter distribution such that micropores having diameters of100-160 Å constitute 70-85% of the total pore volume of the catalyst andmacropores having diameters of greater than 250 Å constitute 5.5-22.0%of the total pore volume of the catalyst.

U.S. Pat. No. 4,670,132 (Arias, et al.) discloses a catalyst preparationand a catalyst composition useful in the hydroconversion of heavy oils,the catalyst comprising a high iron content bauxite with the addition ofone or more of the following promoters: phosphorus, molybdenum, cobalt,nickel or tungsten. The bauxite catalysts typically contain 25-35 wt. %aluminum. The catalysts have certain characteristic features for theelemental components (including aluminum and where present, molybdenum)when the pellet exteriors are examined in the fresh oxide state usingX-ray photoelectron spectroscopy (XPS). For those catalysts whichcontain molybdenum, the surface Mo/Al atomic ratios on the pelletexteriors are in the range of 0.03 to 0.09.

U.S. Pat. No. 4,652,545 (Lindsley, et al.) discloses a catalystcomposition useful in the hydroconversion of heavy oils, the catalystcontaining 0.5-5% Ni or Co and 1.8-18% Mo (calculated as the oxides) ona porous alumina support, such as alumina containing a minor amount ofsilica, having 15-30% of the Ni or Co in an acid extractable form, andfurther characterized by having a TPV of 0.5-1.5 cc/g with a porediameter distribution such that (i) at least 70% TPV is in pores having80-120 Å diameters, (ii) less than 0.03 cc/g of TPV (6% TPV) is in poreshaving diameters of less than 80 Å and (iii) 0.05-0.1 cc/g of TPV (3-20%TPV) is in pores having diameters of greater than 120 Å. Lindsley, etal. is distinguished from the instant invention in that although itteaches that having a proportion of nickel or cobalt contained in itscatalyst in an acid extractable form is advantageous in terms of heavyoil hydroconversion. Lindsley, et al. does not teach or suggest thatcatalysts which have a prescribed molybdenum gradient are advantageousin terms of heavy oil hydroconversion.

U.S. Pat. No. 4,588,709 (Morales, et al.) discloses a catalystpreparation and a catalyst composition useful in the hydroconversion ofheavy oils, the catalyst comprising 5-30 wt. % of a Group VIB element(e.g., molybdenum) and 1-5 wt. % of a Group VIII element (e.g., nickel).Morales, et al. indicate that the finished catalysts have average porediameters of 150 to 300 Angstroms. The catalysts have certaincharacteristic features for the active components (Mo and Ni) when thepellet exteriors are examined in a sulfided state using X-rayphotoelectron spectroscopy (XPS). Morales ('709) requires a largeaverage pore diameter (150 to 300 Angstroms) and Morales ('709) requirescertain characteristic XPS features of the pellet exteriors afterpresulfiding.

U.S. Pat. No. 4,579,649 (Morales, et al.) discloses a catalystpreparation and a catalyst composition useful in the hydroconversion ofheavy oils, the catalyst containing a Group VIB element (e.g.,molybdenum), a Group VIII element (e.g., nickel) and phosphorus oxide ona porous alumina support. The catalyst has certain characteristicfeatures for the three active components (Mo, Ni and P) where the pelletexteriors are examined in a sulfided state using X-ray photoelectronspectroscopy (XPS). Morales ('649) requires certain characteristic XPSfeatures of the pellet exteriors after presulfiding whereas the catalystof the instant invention requires a specified molybdenum gradient asdetermined by measuring the molybdenum/aluminum atomic ratios by XPS forcatalyst pellet exteriors and the pellets in a crushed form as measuredon the fresh catalysts in an oxide state.

U.S. Pat. No. 4,520,128 (Morales, et al.) discloses a catalystpreparation and a catalyst composition useful in the hydroconversion ofheavy oils, the catalyst containing 5-30 wt. % of a Group VIB element(e.g., molybdenum), 0.1-8.0 wt. % of a Group VIII element (e.g., nickel)and 5-30 wt. % of a phosphorus oxide on a porous alumina support. Thefinished catalysts of Morales ('128) have mean pore diameters of 145 to154 Angstroms. The catalyst has certain characteristic features for thethree active components (Mo, Ni and P) when the pellet exteriors areexamined in a sulfided state using X-ray photoelectron spectroscopy(XPS). The catalyst of Morales requires a high phosphorus oxide content.

U.S. Pat. No. 5,047,142 (Sherwood, Jr., et al.) discloses a process ofhydroprocessing a sulfur- and metal-containing hydrocarbon feed whichcomprises contacting said feed with hydrogen and a catalyst in a mannersuch that the catalyst is maintained at isothermal conditions and isexposed to a uniform quality of feed, where said catalyst has acomposition comprising 1.0-5.0 wt. % of an oxide of nickel or cobalt and10.0-25.0 wt. % of an oxide of molybdenum, all supported on a porousalumina support in such a manner that the molybdenum gradient of thecatalyst has a value of less than 6.0, 15-30% of the nickel or cobalt isin an acid extractable form, and said catalyst is further characterizedby having a total surface area of 150-210 m² /g, a total pore volume of0.50-0.75 cc/g, and a pore size distribution such that pores havingdiameters of less than 100 Å constitute less than 25.0%, pores havingdiameters of 100-160 Å constitute 70.0-85.0% and pores having diametersof greater than 250 Å constitute 1.0-15.0% of the total pore volume ofsaid catalyst.

U.S. Pat. No. 4,886,582 (Simpson) discloses a catalyst comprising atleast one metal hydrogenating component comprising Group VIB, such asmolybdenum, or Group VIII metal, such as nickel, on a porous refractoryoxide, such as lithia-alumina, silicaalumina, etc., said compositioncomprising less than 15 wt. % of said metal hydrogenation componentcalculated as the trioxide, and having a pore size distribution where atleast 75% of the total pore volume is in pores of diameters from about20 Angstroms below the pore mode diameter to about 20 Angstroms abovethe pore mode diameter, less than 10% of the total pore volume is inpores of diameters less than 60 Angstroms and greater than 3% to lessthan 10% of the total pore volume is in pores greater than 110 Angstromsand the pore mode diameter is in the range of about 70 to about 90Angstroms.

U.S. Pat. No. 4,846,961 (Robinson, et al.) discloses a hydroprocessingcatalyst containing nickel, phosphorus and about 19 to about 21.5 wt. %of molybdenum (MoO₃) components on a porous refractory oxide such assilica-alumina. The catalyst has a narrow pore size distribution whereinat least 75% of the pore volume is in pores of diameters from about 50to about 110 Angstroms, at least 10% of the pore volume in pores ofdiameters less than 70 Angstroms and at least 60% of the pore volume inpores of diameters within about 20 Angstroms above or below the averagepore diameter. The catalyst is employed to hydroprocess a hydrocarbonoil, especially those oils containing sulfur and nitrogen components.

U.S. Pat. No. 4,686,030 (Ward, et al.) discloses a mild hydrocrackingprocess using a catalyst containing at least one active hydrogenationmetal component supported on an amorphous porous refractory oxide suchas silica-alumina wherein the catalyst has a narrow pore sizedistribution including at least 75% of the total pore volume in pores ofdiameters from about 50 to about 130 Angstroms. Preferably, the catalysthas at least about 60% of the pore volume in pores of diameter withinabout 20 Angstroms above or below a mode pore diameter in the range fromabout 55 to about 100 Angstroms. In one embodiment, a vacuum gashydrocarbon oil is mildly hydrocracked, with simultaneousdesulfurization and denitrogenation, by contact with the catalyst undermild hydrocracking conditions correlated so as to convert about 10 toabout 50 Vol % of the oil fraction boiling above 700° F. to hydrocarbonproducts boiling at or below about 700° F. In other embodiments, thehydrocarbon oil may be desulfurized and denitrogenated either prior toor following the mild hydrocracking.

SUMMARY OF THE INVENTION

The instant invention is a process of mild hydrocracking of a sulfur-and metal-containing hydrocarbon feedstock having a substantialproportion of components boiling below about 1000° F., such as residue,vacuum gas oils, etc., which comprises contacting the feedstock at anelevated temperature and at a pressure of less than 1500 psig withhydrogen and a catalyst which comprises about 2.0 to about 6.0 wt. %,preferably about 2.5 to about 3.5 wt. % of an oxide of a Group VIIImetal, preferably nickel or cobalt; about 12.0 to about 25.0 wt. %,preferably about 12.0 to about 18.0 wt. % of an oxide of molybdenum;about 0 to about 3.0 wt. %, preferably about 0 to about 2.0 wt. % of anoxide of phosphorus, preferably P₂ O₅, all supported on a poroussilica-alumina support containing about 4.0 to about 30.0 wt. %,preferably about 4.0 to about 25.0 wt. %, based on the weight of thesupport, of silica. The molybdenum gradient of the catalyst ranges fromabout 1 to about 10, preferably from about 1 to about 6. This inventionalso related to the catalyst employed in the described process.

In one embodiment of this invention in which the catalyst is preparedusing a silica-alumina support containing about 10 to about 25 wt. ofsilica, the catalyst has a total pore volume in the range of about 0.75to about 0.92 cc/g, and a surface area in the range of about 150 toabout 250 m² /g. The pore volume distribution as determined by mercuryporosimetry consists of about 25 to about 40% of the pore volume inpores with diameters greater than 250 Å, about 30 to about 50% of thepore volume in pores having diameters greater than 160 Å, about 50 toabout 70% of the pore volume in pores with diameters less than 160 Å,and about 20 to about 40% of the pore volume in pores having diametersless than 100 Å. The pore mode of the catalyst as determined by the BETmethod is in the range of 80-120 Å. The preferred pore volumedistribution as determined by mercury porosimetry consists about 28 toabout 35% of the pore volume in pores with diameters greater than 250 Å,about 35 to about 45% of the pore volume in pores having diametersgreater than 160 Å, about 55 to about 65% of the pore volume in poreswith diameters less than 160 Å, and about 24 to about 32% of the porevolume in pores having diameters less than 100 Å. The pore mode of thecatalyst as determined by the BET method is in the range of about 80 toabout 120 Å.

In a second embodiment of this invention in which the catalyst isprepared using a silica-alumina support containing about 4 to about 10%of silica, the catalyst has a total pore volume in the range of about0.60 to about 0.80 cc/g and a surface area in the range of about 150 toabout 250 m² /g. The pore volume distribution consists about 5 to about15% of the pore volume in pores with diameters greater than 250 Å, about10 to about 35% of the pore volume in pores having diameters greaterthan 160 Å, about 65 to about 85% of the pore volume in pores withdiameters less than 160 Å, and about 0 to about 20% of the pore volumein pores having diameters less than 100 Å. The pore mode of the catalystis in the range of about 100 to about 140 Å. The preferred pore volumedistribution consists about 10 to about 15% of the pore volume in poreswith diameters greater than 250 Å, about 20 to about 30% of the porevolume in pores having diameters greater than 160 Å, about 70 to about80% of the pore volume in pores with diameters less than 160 Å, and 0 toabout 10% of the pore volume in pores having diameters less than 100 Å.The pore mode of the catalyst is in the range of about 100 to about 140Å.

The use of the catalysts of this invention not only provides ahydrocarbon conversion advantage but also maintains the sediment-make ata level similar to the conventional bimodal alumina based catalysts. Theinstant invention is much improved over the prior art catalysts in termsof sediment formation and reactor unit operability.

The operating conditions for the process of the instant invention aresuch as to yield about a 10 to about a 60 vol % conversion of thehydrocarbon feedstock boiling at 650° F.+ to hydrocarbon productsboiling at 650° F.-.

The residuum feedstocks may be contacted with hydrogen and the catalystutilizing a wide variety of reactor types. Preferred means for achievingsuch contact include contacting the feed with hydrogen and theprescribed catalyst in a fixed bed hydrotreater, in a singlecontinuous-stirred-tank reactor or single ebullated-bed reactor, or in aseries of 2-5 continuous-stirred-tank or ebullated-bed reactors, withebullated-bed reactors being particularly preferred. The process of theinstant invention is particularly effective in achieving high conversionrates of 1000° F.+ to 1000° F.- fractions while maintaining desirablelevels of 650° F.+ conversion, sediment make and HDS activity comparedto the results obtained with the use of conventional bimodalalumina-based catalysts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The decreasing demand for heavy fuel oils has caused refiners to seekways to convert heavier hydrocarbon feedstocks to lighter products ofmore value. To increase mid-distillate production, the refiner hasseveral process options. They include hydrocracking, fluid catalyticcracking, and coking, which all require heavy investments in therefineries. Because of such high costs, refiners are continuallysearching for conversion processes which may be utilized in existingunits. An additional option available to refiners is to employ a mildhydrocracking (MHC) process. MHC process is an evolution of the VGOhydrodesulfurization (HDS) process. The main feedstock for this MHCprocess is VGO but other types of heavy gas oils, such as coking gasoils and deasphalted oils, can be used.

The major advantage of MHC is that it can be carried out within theoperating constraints of existing VGO hydrotreaters. The typicalconditions for the MHC process are: Temperature: 720°-780° F., HydrogenPressure: 600-1200 psig, H₂ /Oil Ratio: 1000-2000 SCF/BBL, SpaceVelocity: 0.4-1.5 Vol/Vol/Hr. In contrast, true high conversionhydrocracking units are operated at these conditions: Temperature:700°-900° F., Hydrogen Pressure: 1800-3000 psig, H₂ /Oil Ratio:1400-6000 SCF/BBL, Space Velocity: 0.3-1.5 Vol/Vol/Hr. The majordifference between the two processes is the hydrogen pressure.

The products obtained from the MHC process are low sulfur fuel oil(60-80%) and middle distillate (20-40%). This hydrotreated fuel oil isalso an excellent feed for catalytic cracking because of its higherhydrogen content and lower nitrogen content compared to the originalfeed. The quality of diesel cut produced by MHC is usually close todiesel oil specifications for the cetane index, and so can be added tothe diesel pool.

The switch from a HDS mode to a MHC mode can be achieved in differentways, assuming that the refiner is equipped to recover the surplus ofthe middle distillate fraction. One way to increase middle distillateproduction from a unit loaded with HDS catalyst is to increase theoperating temperature. Using a conventional hydrotreating catalyst, theMHC process typically converts about 10 to 30 Vol % of hydrocarbonfeedstock boiling above 650° F. (650° F.+) to middle distillate oilsboiling at or below 650° F. (650° F.-).

Another way to increase the middle distillate production is to change,at least partly, a HDS catalyst on a nonacidic alumina support to aslightly acidic catalyst. Catalysts of higher activity are still beingsought. The higher the activity of the catalyst, the lower thetemperature required to obtain a product of given sulfur, nitrogen ormetal content in any given boiling range. For the VGO containing a highproportion of residuum, an HDS catalyst usually gives less than 10 Vol %conversion of the 650° F.+ fraction. The conversion of resid componentsboiling above 1000° F. (1000° F.+) into products boiling at or below1000° F. (1000° F.-) with the known alumina-based hydrotreatingcatalysts is achieved primarily by thermal cracking reactions.

A particular difficulty which arises in resid hydroprocessing unitsemploying the currently known catalysts is the formation of insolublecarbonaceous substances (also called sediment) when the conversion ishigh (above 50 Vol %). High sediment may cause plugging of reactor ordownstream units, such as a fractionation unit. The higher theconversion level for a given feedstock, the greater the amount ofsediment formed. This problem is more acute at a low hydrogen pressureand high reaction temperature.

The process of the instant invention employs a catalyst compositioncomprising about 2.0-6.0, preferably 2.5-3.5 wt. % of an oxide of aGroup VIII metal, preferably nickel or cobalt, most preferably NiO about12.0 to about 25.0 wt. %, preferably about 12.0 to about 18.0 wt. % ofan oxide of molybdenum, most preferably MoO₃ and about 0 to about 3.0,preferably 0 to about 2.0 wt. % of an oxide of phosphorus, preferably P₂O₅ all supported on a porous silica-alumina support containing about 4.0to about 30.0 wt. %, preferably about 4.0 to about 25.0 wt. %, based onthe weight of the support, of silica. Most preferably, the support isgamma alumina. Groups VIII, as referred to herein, is Groups VIII of thePeriodic Table of Elements. The Periodic Table of Elements referred toherein is found on the inside cover of the CRC Handbook of Chemistry andPhysics, 55th Ed. (1974-75). Other oxide compounds which may be found insuch a catalyst composition include SO₄ (present in less than 0.8 wt.%), and Na₂ O (present in less than 0.1 wt. %). The above-describedsilicaalumina support may be purchased or prepared by methods well knownto those skilled in the art.

Catalyst Preparation

In preparing the catalyst the support containing silica is impregnatedwith the requisite amounts of the VIB metal oxide and Group VIII metaloxide and, optionally, phosphorus oxide via conventional means known tothose skilled in the art to yield a finished catalyst containing a GroupVIII metal oxide in the amount of 2.0 to about 6.0 wt. %, preferablyabout 2.5 to about 3.5 wt. %, molybdenum oxide in the amount of 12.0 toabout 25.0 wt. %, preferably 12.0 to about 18.0 wt. % and phosphorusoxide in the amount of about 0 to about 3.0 wt. %, preferably 0 to about0.1 wt. %.

The Group VIII metal may be iron, cobalt, or nickel which is loaded onthe support, for example, as a 10-30 wt. %, preferably about 15 wt. % ofan aqueous solution of metal nitrate. The preferred metal of this groupis nickel which may be employed at about 16 wt. % aqueous solution ofnickel nitrate hexahydrate. Molybdenum may be loaded on the supportemploying, for example, a 10-20 wt. %, preferably about 15 wt. %, of anaqueous solution of ammonium heptamolybdate (AHM). The phosphoruscomponent, when utilized, may be prepared from 85% phosphoric acid.

The active metals and phosphorus may be loaded onto the catalyst supportvia pore filling. Although it is possible to load each metal separately,it is preferred to impregnate simultaneously with the Group VIII metaland molybdenum compounds, phosphoric acid, as well as with stabilizerssuch as hydrogen peroxide and citric acid (monohydrate), when employed.It is preferred that the catalyst be impregnated by filling 95-105%, forexample, 97% of the support pore volume with the stabilized impregnatingsolution containing the required amount of metals and citric acid.

Finally, the impregnated support is oven-dried and then directlycalcined preferably at 1000°-1150° F. for about 20 minutes to 2 hours inflowing air.

A hydroconversion process, such as a mild hydrocracking process, whichpreferentially removes sulfur and nitrogen from the converted productstream with components having boiling points less than 1000° F. isdesirable in those instances where there is less concern over thequality of the unconverted product stream, but, rather, where theprimary concern is the quality of the distillate product from thehydroconversion process. It is well known to those skilled in the artthat high heteroatom contents of distillate hydroconversion productshave an adverse effect on fluid catalytic cracking of the heavier gasoils (having a boiling point of about 650° F. to about 1000° F.) andthat extensive hydrotreating of the distillate streams would be requiredto meet the strict mandated levels of heteroatoms in distillate fuels.The demands placed upon catalyst compositions make it difficult toemploy a single catalyst in a hydroconversion process, such as a mildhydrocracking process, which will achieve effective levels of sulfur andnitrogen removal from the converted product stream having componentswith boiling points below 1000° F. However, the catalyst employed in theprocess of the instant invention is capable of achieving such resultsbecause the prescribed catalyst has an optimized micropore diameter toovercome the diffusion limitations for hydrotreatment of the convertedproduct molecules but it also does not contain such large macroporesthat would allow poisoning of the catalyst pellet interior. Aspreviously described, the catalyst also has a specified pore sizedistribution such that pores with diameters less than 55 Å are minimizedas these pores are easily plugged with contaminants duringhydroprocessing.

Catalyst Examples SN-6599, 6600, 6601, 6616 and 6602, the properties ofwhich are described in Table I below, as well as Catalyst ExamplesSN-6922, 6923 and 6615, the properties of which are described in TableII below, are catalysts prepared in the manner set out above, which maybe employed in the process of this invention while the properties of theSupport SN-6599X used in processing Catalyst SN-6599, Support SN-6602Xused in preparing Catalyst SN-6602 and Support SN-6923X employed inpreparing Catalyst SN-6923 are IS described in Tables III and IV below.The catalysts were prepared with a commercially available silicaaluminasupport obtained from American Cyanamid and are available in the form ofextrudates in the diameter range of 0.035-0.041 inch.

The silica content of the catalysts described in Tables I and II isbased on the weight of the catalyst support.

                                      TABLE I                                     __________________________________________________________________________    NiMo CATALYSTS ON SILICA-ALUMINA SUPPORTS                                                Catalyst                                                                      SN-6599                                                                            SN-6600                                                                            SN-6601                                                                            SN-6616                                                                            SN-6602                                        __________________________________________________________________________    Impreg. Sol'n.                                                                           NiMo NiMo NiMo NiMo NiMo                                                      (H.sub.2 O.sub.2)                                                                  (H.sub.2 O.sub.2)                                                                  (H.sub.2 O.sub.2)                                                                  (Citric                                                                            (Phosphoric                                                              Acid)                                                                              (Acid)                                         SiO.sub.2 wt. %                                                                          4    8    16   16   8                                              P.sub.2 O.sub.5 wt. %                                                                    0    0    0    0    1.6                                            NiO Wt. %  3.3  3.3  3.2  3.2  3.2                                            MoO.sub.3 wt. %                                                                          14.1 13.3 11.4 14.7 13.5                                           Pore Volume Distribution by Hg Porosimetry; Surface Area by Nitrogen BET      Total PV, cc/g                                                                           0.64 0.77 0.85 0.79 0.67                                           PV > 250Å % TPV                                                                      10.7 14.3 31.8 32.9 9.0                                            PV > 160Å % TPV                                                                      14.1 32.5 39.8 41.8 22.4                                           PV < 160Å % TPV                                                                      84.4 67.5 61.2 59.4 76.1                                           PV < 100Å % TPV                                                                      15.6 5.2  31.8 35.9 7.5                                            PV < 55Å % TPV                                                                       0    0    0.5  0    0                                              PV 100-160Å % TPV                                                                    70.3 62.3 30.6 31.6 68.7                                           PM at (dv/dD) max Å                                                                  115  138  83   79   132                                            PM (BET), Å                                                                          106  138  88   68   129                                            Surf. Area, m.sup.2 /g                                                                   194  171  198  193  163                                            HDS-MAT, C.sub.0.5g, %                                                                   93   87   64   57   69                                             Metals Distribution by XPS Analysis                                           (Mo/Al).sub.int                                                                          0.12 0.10 0.08 0.08 0.11                                           (Ni/Al).sub.int                                                                          0.013                                                                              0.015                                                                              0.012                                                                              0.010                                                                              0.018                                          Mo Gradient                                                                              6.0  5.4  7.9  1.12 5.8                                            Ni Gradient                                                                              1.4  1.1  1.8  1.2  1.4                                            __________________________________________________________________________

                  TABLE II                                                        ______________________________________                                        NiMo CATALYSTS ON SILICA-ALUMINA SUPPORTS                                                Catalyst                                                                      SN-6922  SN-6923  SN-6615                                          ______________________________________                                        Impreg. Sol'n.                                                                             Ni--Mo     Ni--Mo   Ni--Mo                                                    (Citric    (Citric  (Phosphoric                                               Acid)      Acid)    Acid)                                        SiO.sub.2 wt. %                                                                            16         16       16                                           P.sub.2 O.sub.5 wt. %                                                                      0          0        1.6                                          MoO.sub.3 wt. %                                                                            14.2       14.2     15.0                                         NiO Wt. %    3.1        2.8      3.2                                          Total PV, % TPV                                                                            0.88       0.82     0.78                                         PV > 250Å % TPV                                                                        28.4       31.7     33.3                                         PV > 160Å % TPV                                                                        36.4       40.2     42.3                                         PV < 55Å % TPV                                                                         2.2        1.9                                                   PV < 160Å % TPV                                                                        63.6       59.8     57.7                                         PV < 100Å % TPV                                                                        35.2       24.4     25.6                                         PV 100-160Å                                                                            28.4       34.1     32.1                                         PM at (dv/dD) max Å                                                                    93         110      88                                           PM (BET), Å                                                                            98         109      84                                           Surf. Area, m.sup.2 /g                                                                     239        193      173                                          HDS-MAT, C.sub.0.5g, %                                                                     92         84       50                                           Metals Distribution by XPS Analysis                                           (Mo/Al).sub.int                                                                            0.11       0.10     0.10                                         (Ni/Al).sub.int                                                                            0.010      0.009    0.010                                        Mo Gradient  2.2        3.7      1.4                                          Ni Gradient  1.8        3.1      1.3                                          ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        PROPERTIES OF SILICA-ALUMINA SUPPORTS                                                    Catalyst Support                                                              SN-6599X SN-6602X  SN-6923X                                        ______________________________________                                        SiO.sub.2 wt. %                                                                            4          8         16                                          NH.sub.3 Desorbed, cc/g                                                                    11.8       10.6      11.1                                        Pore Volume Distribution by Hg Porosimetry                                    TPV, cc/g    0.81       0.85      1.04                                        PV > 250Å % TPV                                                                        8.6        11.8      24.0                                        PV > 160Å % TPV                                                                        14.8       25.9      40.4                                        PV < 160Å % TPV                                                                        85.2       75.3      59.6                                        PV < 100Å % TPV                                                                        19.8       14.1      20.2                                        PV 100-160Å % TPV                                                                      65.4       61.2      39.4                                        MPD (Vol), Å                                                                           117        128       102                                         TSA (N.sub.2), m.sup.2 g                                                                   N/A        187       N/A                                         ______________________________________                                         Note: The suffix X denotes the support for the catalyst of the same           number. The contact angle used in the determination of Median Pore Mode b     Volume denoted as MPD (Vol) for the supports and the finished catalysts       was 140 and 130 degrees, respectively.                                   

                                      TABLE IV                                    __________________________________________________________________________    NiMo CATALYSTS ON SILICA-ALUMINA SUPPORTS                                                SN-6616X                                                                            SN-6616                                                                            SN-6600X                                                                            SN-6600                                                                            SN-6922X                                                                            SN-6922                                __________________________________________________________________________    Impreg. Sol'n.                                                                           Support                                                                             Catalyst                                                                           Support                                                                             Catalyst                                                                           Support                                                                             Catalyst                                                Ni--Mo     Ni--Mo     Ni--Mo                                                  (Citric    (H.sub.2 O.sub.2)                                                                        (Citric                                                 Acid)                 Acid)                                  NH.sub.3 Desorbed, cc/g                                                                  4.7        11.0       11.1                                         SiO.sub.2, wt. %                                                                         16         8.0        16                                           MoO.sub.3, wt. % 14.7       13.3       14.2                                   NiO, wt. %       3.2        3.3        3.1                                    Pore Volume Distribution by Hg Porosimetry                                    TPV, cc/g  1.04  0.79 0.99  0.77 1.06  0.88                                   PV > 250Å % TPV                                                                      32.7  31.6 15.2  14.3 30.2  28.4                                   PV > 150Å % TPV                                                                      41.3  35.4 34.3  32.5 36.8  36.4                                   PV < 160Å % TPV                                                                      58.7  59.5 65.7  67.5 63.2  63.6                                   PV < 100Å % TPV                                                                      36.8  30.4 9.1   5.2  40.6  36.2                                   PV 100-160Å % TPV                                                                    27.9  29.1 56.6  62.3 22.6  28.4                                   as % of PV < 250Å                                                                    41    43   67    73   32    39                                     MBD (Vol), Å                                                                         104   105  114   N/A  87    93                                     TSA (N.sub.2), m.sup.2 /g                                                                212   187  N/A   171  N/A   239                                    __________________________________________________________________________     Note: The suffix X denotes the support for the catalyst of the same           number. The contact angle used in the determination of Median Pore Mode b     Volume denotes as MPD (Vol) for the supports and the finished catalyst wa     140 and 130 degrees, respectively.                                       

The properties of two commercially available hydroprocessing catalystsare set forth in Table V below. All of these catalysts are availablestate of the art catalysts sold for use in hydroprocessing resid oils.Catalyst A, which is American Cyanamid HDS-1443B catalyst, is referredto in this specification as the standard reference catalyst.

Pore structure values set out in Tables I-V were determined usingMicrometrics Autopore 9220 Mercury Porosimetry Instrument. In the tablesreference to BET means values measured by the Brunauer-Emmett-TellerTechniques.

                  TABLE V                                                         ______________________________________                                        ALUMINA BASED                                                                 CATALYSTS AS CONTROL EXAMPLES                                                                 Catalyst                                                                      A      B                                                      ______________________________________                                        Impreg. Sol'n.    Ni--Mo   Ni-Mo                                              MoO.sub.3 wt. %   11.5-14.5                                                                              14.5-15.5                                          NiO wt. %         3.2-4.0  3.0-3.5                                            Pore Volume Distribution by Hg Porosimetry                                    Total PV, cc/g    0.74     0.64                                               PV > 250Å % TPV                                                                             33.8     7.8                                                PV > 160Å % TPV                                                                             37.8     17.2                                               PV < 160Å % TPV                                                                             62.2     84.4                                               PV < 100Å % TPV                                                                             58.1     9.4                                                PV 100-160Å % TPV                                                                           4.1      75.1                                               PM at (dv/dD) max Å                                                                         50       126                                                PM (BET), Å   46       105                                                Surf. Area, m.sup.2 /g                                                                          314      194                                                HDS-MAT, C.sub.0.5g, %                                                                          73       88                                                 Metals Distribution by XPS Analysis                                           (Mo/Al).sub.int   0.09     0.012                                              (Ni/Al).sub.int   0.012    0.016                                              Mo Gradient       1.2      3.1                                                Ni Gradient       1.6      1.0                                                ______________________________________                                    

A preferred feature of the catalyst composition of the instant inventionis that the above-described oxide of molybdenum, preferably MoO₃, isdistributed on the above-described porous alumina support in such amanner that the molybdenum gradient of the catalyst has a value of about1.0 to about 10.0, As used in this description and in the appendedclaims, the phrase "molybdenum gradient" means that the ratio of a givencatalyst pellet exterior molybdenum/aluminum atomic ratio to a givencatalyst pellet interior molybdenum/aluminum atomic ratio has a value ofless than 6.0, preferably 1.0-5.0, the atomic ratios being measured byX-ray photoelectron spectroscopy (XPS), sometimes referred to asElectron Spectroscopy for Chemical Analysis (ESCA). It is theorized thatthe molybdenum gradient is strongly affected by the impregnation ofmolybdenum on the catalyst support and the subsequent drying of thecatalyst during its preparation. ESCA data on both catalyst pelletexteriors and interiors were obtained on an ESCALAB MKII instrumentavailable from V. G. Scientific Ltd., which uses a 1253.6 electron voltmagnesium X-ray source. Atomic percentage values were calculated fromthe peak areas of the molybdenum 3_(p3/2) and aluminum 2_(p3/2) signalsusing the sensitivity factors supplied by V. G. Scientific Ltd. Thevalue of 74.7 electron volts for aluminum was used as a referencebinding energy.

To determine the molybdenum/aluminum atomic ratio of a given catalystpellet exterior for the catalyst of the instant invention, the catalystpellets were stacked flat on a sample holder, and subjected to ESCAanalysis. For the catalyst of the instant invention themolybdenum/aluminum atomic ratio of the catalyst pellet exterior is inthe range of 0.12-0.75, preferably 0.12-0.42. This exteriormolybdenum/aluminum atomic ratio is considerably greater than the Mo/Alcatalyst surface atomic ratio of 0.03-0.09 disclosed in U.S. Pat. No.4,670,132.

To determine the molybdenum/aluminum atomic ratio of a given catalystpellet interior for the catalyst of the instant invention, the catalystpellets were crushed into a powder, placed firmly in a sample holder,and subjected to ESCA analysis. For the catalyst of the instantinvention, the molybdenum/aluminum atomic ratio of the catalyst pelletinterior (i.e., the molybdenum/aluminum ratio of the powder, which isassumed to be representative of the interior portion of the pellet) isin the range of 0.08-0.15, preferably 0.11-0.12.

The molybdenum/aluminum atomic ratios of the total catalyst compositionof the instant invention, as determined by conventional means (i.e.,Atomic Absorption (AA) or Inductively Coupled Plasma (ICP)spectroscopies) is in the range of 0.060-0.075, preferably 0.062-0.071.To determine the total catalyst composition molybdenum/aluminum atomicratio, catalyst pellets were ground to a powder and digested in acid toform an ionic solution. The solution was then measured by AA or ICP todetermine Mo ion concentration, which was then adjusted to MoO₃concentration. Alumina (Al₂ 0₃) concentration was back-calculated fromthe direct measurement of the concentrations of the other components(e.g., Ni, Fe, Na, S).

The HDS Microactivity Test (HDS-MAT) was used to evaluate the intrinsicactivity of catalysts in the absence of diffusion and using a modelsulfur compound as a probe. The catalyst, ground to a 30-60 meshfraction, is presulfided at 750° F. with a 10% H₂ S/H₂ mixture for 2hours. The presulfided catalyst is exposed to abenzothiophene-containing feed at 550° F. and flowing hydrogen forapproximately four hours. Cuts are taken periodically and analyzed by agas chromatograph for the conversion of benzothiophene to ethylbenzene.The results obtained with HDS-MAT tests as well as the Mo and Nigradients of the catalysts described are shown in Tables I and II.

Catalyst Properties

Tables III and IV show the pore volume distributions and surface areasof six silica-alumina supports with silica contents varying from 4 to 16wt. %. As the silica content was increased from 4 to 16%, the total porevolume (TPV) and the macroporosity (PV>250 Å) increased, whereas thepore volume in the region of PV 100-160 Å decreased. The TPV of thesupports was in the range of 0.81 to 1.06 cc/g and the macroporosity wasvaried from 0.07 to 0.34 cc/g. The pore volume of pores with diametersin the range of 100-160 Å was varied from 0.24 to 0.56 cc/g or 32 to 72%of the pore volume of pores having diameters less than 250 Å. Acomparison of the pore volume distributions between the support and thefinished catalyst indicated that the pore volume in the region of PV100-160 Å was maintained essentially the same after impregnation ofactive metals.

There is another unique feature of this type of silica-alumina incontrast to the conventional silica-alumina used in the cracking andhydrocracking catalysts. That is, the acidity of the supports, shown inTable III as NH₄ desorbed, does not depend on the silica content. Themethod employed to incorporate silica sol into the alumina gel is noteffective for enhancing the acidity function of the support and thefinished catalysts.

The properties of five silica-alumina based NiMo catalysts are comparedin Table I. Three kinds of impregnation stabilizers, hydrogen peroxide,citric acid and phosphoric acid, were used in the co-impregnation of Niand Mo onto the silica-alumina supports. It is seen that the HDS-MATactivity decreases with increasing silica content. All of the catalystslisted in Table I except the citric acid stabilized SN-6616 show high Mogradients. To improve the HDS-MAT activity of NiMo catalysts onsilica-alumina supports of 16% silica, the calcination temperature waslowered to 1100° F. (in the cases of SN-6922 and SN-6923). As seen inTable II, the HDS-MAT activities of SN-6601 and SN-6923 aresignificantly higher than SN-6601 and SN-6615. By contrast, the Mogradients of SN-6922 and SN-6923 are significantly lower than that ofSN-6601. Therefore, the citric acid is the most effective impregnationstabilizer to achieve high dispersion of Mo and uniform laydown of Moacross the catalyst extrudates.

BERTY REACTOR HYDROCRACKING CATALYST EVALUATION

The Berty reactor, a type of continuous stirred tank reactor (CSTR), wasused to determine hydrocracking activities of the catalysts of thisinvention in a diffusion controlled regime at a low rate ofdeactivation. The catalysts were presulfided and then the reaction wascarried out at a single space velocity for 38 hours. The sample cutswere taken every 4 hours and tested for boiling point distribution, Ni,V, S, and sediment content. Using these data, conversions for the 650°F.+ and 1000° F.+ fractions were determined. The feedstock propertiesand the operating conditions of the Berty reactor are listed in Table VIwhich follows.

The hydrocracking activity was determined by comparing the percentagesof products in the 650° F.- fraction and 1000° F.- fraction when variouscatalysts were evaluated under constant mild hydrocracking conditionswith the same feedstock. The conversions of 650° F.+ and 1000° F.+ werecalculated by the equations below: ##EQU1## Y(F) denotes the volumepercentage of the 650° F.+ or 1000° F.+ fraction in the feedstock.

Y(P) denotes the volume percentage of the 650° F.+ or 1000° F.+ fractionin the products.

The boiling point distribution of the total product was determined usingthe ASTM D-2887 Method, Simulated Distillation by Gas Chromatography.The existent sediment content in the total product was measured by usingthe IP 375/86 Method, Total Sediment in Residual Fuels. The TotalSediment is the sum of the insoluble organic and inorganic materialwhich is separated from the bulk of the residual fuel oil by filtrationthrough a filter medium, and which is also insoluble in a predominantlyparaffinic solvent.

                  TABLE VI                                                        ______________________________________                                        BERTY REACTOR OPERATING CONDITIONS                                            ______________________________________                                        1. PRESULFIDING                                                               Temperature       750°-800° F.                                  Pressure          40 Psig                                                     Gas Mixture       10 Vol % H.sub.2 S - 90 Vol % H.sub.2                       Gas Flow          500 sccm                                                    Duration          2 Hr., 45 Min.                                              2. FEEDSTOCK                                                                                    60 Vol % Desulfurized VGO                                                     40 Vol % Ar M/H Vac. Resid                                  Boiling Point     IBP         444° F.                                  Distribution      FBP         1371° F.                                                   650° F.+                                                                           89.2 Vol %                                                        900° F.+                                                                           45.6 Vol %                                                        1000° F.+                                                                          33.5 Vol %                                      Sulfur wt %       2.2                                                         Ni Content, ppm   20                                                          V Content, ppm    54                                                          3. REACTION CONDITIONS                                                        Temperature       805° F.                                              Pressure          1000 Psig                                                   Hydrogen Feed Rate                                                                              300 SCCM                                                    Liquid Feed Rate  82.5 CC/HR                                                  Liquid Holdup     125 CC                                                      Catalyst Charge   36.9 Grams                                                  ______________________________________                                    

Data listed in Table VII, which follows, show the activity resultsachieved with Catalysts SN-6599, 6600, 6601 and 6616 of this inventioncompared to the activities exhibited by Catalyst A (the referencecatalyst) and Catalyst B, which are both commercial hydroprocessingcatalysts, as determined in the Berty Reactor tests.

The data presented in Table VII show that Catalysts SN-6599, 6600, 6601and 6616, catalysts of this invention, have a substantially greateractivity for the 650° F.+ conversion value than Catalyst A; thatCatalysts SN-6600 and 6616 exhibit a greater activity for the 650° F.+conversion than Catalyst B while Catalyst SN-6601 has a 650° F.+conversion value about equal to Catalyst B and Catalyst SN-6599 exhibitsa 650° F.+ conversion value somewhat less than Catalyst B. CatalystsSN-6599, 6600, 6601 and 6616 all show a 1000° F.+ conversion valuegreater than Catalysts A or B.

                  TABLE VII                                                       ______________________________________                                        BERTY RESID MILD HYDROCRACKING ACTIVITIES                                            650° F.+                                                                          1000° F.+                                                                        IP      HDM   HDS                                        Conversion Conversion                                                                              Sediment                                                                              Act.  Act.                                Catalyst                                                                             Vol %      Vol %     ξ    %     %                                   ______________________________________                                        A      29         78        0.7     80    69                                  B      45         83        0.9     60    71                                  *SN-6599                                                                             40         86        0.6     62    68                                  *SN-6600                                                                             48         91        0.6     81    72                                  *SN-6601                                                                             44         89        0.5     78    55                                  *SN-6616                                                                             48         92        0.4     72    62                                  ______________________________________                                         Run conditions: Temperature = 805° F., Pressure = 1000 psig, LHSV      0.66, Hydrogen Flow Rate = 300 scc/m, and the feedstock is 40 Vol %           Arabian Medium/Arabian Heavy (65:35 Vol %) vacuum resid in desulfurized       vacuum gas oil.                                                               *Catalyst of the instant invention.                                      

Two commercial alumina-based hydroprocessing catalysts, Catalyst A(i.e., Catalyst HDS-1443B) and Catalyst B were used as the reference inthe evaluation for MHC activities. A comparison of conversion advantagesover Catalyst A which has a bimodal pore structure is set out in thedata presented in Table VIII which follows.

                  TABLE VIII                                                      ______________________________________                                        BERTY RESID MILD HYDROCRACKING ACTIVITIES                                              650° F.+                                                                            1000° F.+                                                                        IP                                                     Conversion   Conversion                                                                              Sediment                                               Advantage    Advantage Delta                                         Catalyst Vol %        Vol %     %                                             ______________________________________                                        A          0            0       0                                             B        +16           +5       +0.2                                          *SN-6599 +11           +8       -0.1                                          *SN-6600 +19          +13       -0.1                                          *SN-6601 +15          +11       -0.2                                          *SN-6616 +19          +14       -0.3                                          ______________________________________                                         Run conditions: Temperature = 805° F., Pressure = 1000 psig, LHSV      0.66, Hydrogen Flow Rate = 300 scc/m, and the feedstock is 40 Vol %           Arabian Medium/Arabian Heavy (65:35 Vol %) vacuum resid in desulfurized       vacuum gas oil.                                                               *Catalyst of the instant invention.                                      

The data presented in Table VIII show that Catalysts SN-6599, 6600, 6601and 6616, catalysts of the instant invention, exhibit an increase of 11to 19 Vol % in 650° F.+ conversion which corresponds to a 37.9 to 65.5%improvement in relative conversion over that achieved with Catalyst A(i.e., the standard base commercial catalyst). Catalysts SN-6599, 6600,6601 and 6616 also gave an appreciable improvement in the 1000° F.+conversion ranging from 8 to 14 Vol % which corresponds to a 10.3 to17.9% improvement over that achieved with Catalyst A. The IP sedimentmake of these same catalysts showed a decrease of 0.1 to 0.3% over thesediment make of Catalyst A.

The results set out in Table VIII clearly indicate that thesilica-alumina based catalyst substantially outperforms Catalyst A or Bof the prior art.

Mild hydrocracking of heavy oils containing resids in the presence ofthe catalyst of this invention comprising, for example, molybdenumoxide, nickel oxide, and, optionally, phosphorus oxide on thesilica-alumina support having a specified pore size distribution notonly allows an increased production of middle distillate and moreeffective conversion of resid feedstocks but also maintains the sedimentmake at a low level or similar to that achieved with conventionalbimodal alumina-based catalysts.

What is claimed is:
 1. A catalyst useful for mild hydrocracking of ahydrocarbon feedstock having a substantial proportion of componentsboiling below about 1000° F. comprising about 2.0 to about 6.0 wt. % ofan oxide of a Group VIII metal; about 12.0 to about 25.0 wt. % of anoxide of molybdenum and 0 to about 3.0 wt. % of an oxide of phosphorusall supported on a porous alumina support containing about 10.0 to about25.0 wt. %, based on the weight of the support; of silica in such amanner that the molybdenum gradient of the catalyst has a value betweenabout 1 and about 10 and wherein the catalyst is further characterizedby having a total surface area of about 150 to about 250 m² /g and atotal pore volume of about 0.75 to about 0.92 cc/g with a pore diameterdistribution such that pores having diameters of less than 100 Åconstitute about 20.0 to about 40.0%, pores having diameters of 100-160Å constitute 28.4 to 34.1%; pores having diameters less than 160 Åconstitute about 50.0 to about 70.0%, pores having diameters greaterthan 160 Å constitute about 30.0 to about 50.0%, of the total porevolume of the said catalyst and macropores having diameters greater than250 Å constitute about 25.0 to about 40.0% of the total pore volume ofsaid catalyst.
 2. The catalyst of claim 1 wherein the said catalystcontains about 0.1 to about 2.0 wt. % of phosphorus oxide.
 3. Thecatalyst of claim 1 wherein the said Group VIII metal is selected fromthe group consisting of nickel and cobalt.
 4. The catalyst of claim 1wherein the said Group VIII metal is nickel.
 5. The catalyst of claim 1wherein the said Group VIII metal is cobalt.
 6. The catalyst of claim 1wherein the said catalyst comprises about 2.5 to about 3.5 wt. % NiO andabout 12.0 to about 18.0 wt. % MoO₃ supported on the said porous aluminasupport containing about 16 to about 25.0 wt. % of silica based on theweight of the support.
 7. The catalyst of claim 1 wherein the molybdenumgradient of the catalyst has a value between about 1 and about 10.